It is well known that polymers can be made electrically conductive by dispersing therein suitable amounts of conductive particulate fillers such as carbon black or fine metal particles. Over recent years, there has been particular interest in such compositions that exhibit positive temperature coefficient (PTC) characteristics, i.e., which show a very rapid increase in resistivity over a particular temperature range.
PTC materials are conductive materials characterized by a sharp increase in resistivity upon reaching a switching temperature (Ts). If the jump in resistivity is sufficiently high, the resistivity effectively blocks the current and further heating of the material such that overheating of the material is prevented. One of the main benefits of PTC materials is that no additional electronic circuits are necessary in an article that includes a PTC material since the PTC material itself has a characteristic similar to electronic circuits. Moreover, upon cooling, the material resets itself. This jump in resistivity may oftentimes be referred to as the PTC amplitude and may be defined as the ratio of the maximum volume resistivity to the volume resistivity at room temperature (app. 23° C.).
PTC compositions have been used as a circuit protection device for limiting the current flow when a short-circuiting has taken place in the circuit comprising a heater, a positive character thermistor, a thermo-responsive sensor, a battery or the like, and for resetting the circuit to a normal state when the cause of the short-circuiting is removed.
An alternative use of PTC compositions is a PTC device, in which more than two electrodes are electrically connected to the PTC composition. The electrodes are connected to a power supply so that the current can flow through the PTC device. The PTC device is used as a protecting device for a circuit from current overload, overheating and the like, by functioning as a self-temperature controller as described above. By functioning as a self-controller point towards the use as a self-controlled heater rather than circuit protector.
Compositions exhibiting PTC behavior have been used in electrical devices as over-current protection in electrical circuits comprising a power source and additional electrical components in series. Under normal operating conditions in the electrical circuit, the resistance of the load and the PTC device is such that relatively little current flows through the PTC device. Thus, the temperature of the device remains below the critical or trip temperature. If the load is short circuited or the circuit experiences a power surge, the current flowing through the PTC device increases greatly. At this point, a great deal of power is dissipated in the PTC device. This power dissipation only occurs for a short period of time (fraction of a second), however, because the power dissipation will raise the temperature of the PTC device to a value where the resistance of the PTC device has become so high, that the current is limited to a negligible value. The device is said to be in its “tripped” state. The negligible or trickle through current that flows through the circuit will not damage the electrical components which are connected in series with the PTC device. Thus, the PTC device acts as a form of a fuse, reducing the current flow through the short circuit load to a safe, low value when the PTC device is heated to its critical temperature range. Upon interrupting the current in the circuit, or removing the condition responsible for the short circuit (or power surge), the PTC device will cool down below its critical temperature to its normal operating, low resistance state. The effect is a resettable, electrical circuit protection device.
PTC materials have also been utilized in self-controlled heaters. When connected to a power source, the PTC material will heat up to the trip temperature and maintain this temperature without the use of any additional electronic controllers.
U.S. Pat. No. 6,479,575 discloses the use of two types of polymers that have either similar or dissimilar molecular structures and preferably at least two types of carbon blacks, and to electrical devices containing such compositions. U.S. Pat. No. 6,277,303 discloses an immiscible blend of at least two polymers that phase separate into two continuous morphologies and invention exploits the aspects of percolation theory in developing very low conductive filler content conductive polymeric materials.
Most polymeric PTC materials are based on semi-crystalline polymers with conductive fillers like carbon black, graphite, metal powders and or ceramic powders. One of the draw back of these systems is that the PTC effect occurs at or close to the onset of melting of polymer which results in the volume expansion and the corresponding dilution of the conducting filler in the polymer. The application of these PTC materials is limited as the heat deflection temperature of these materials is less than the trip temperature of the PTC material.
The present invention improves on the prior art by investigating the need for a composition exhibiting positive temperature coefficient (PTC) that has high distortion temperatures. Applicants have found that this can be achieved by the use of a co-continuous blend composition in which one of the phases is a PTC material and the other phase a non-conductive composition with a heat distortion temperature higher than the trip temperature of the PTC material, characterized that the co-continuous blend composition has a heat distortion temperature higher than the PTC trip temperature.